1. Field of the Invention
The field of the invention is that of start up circuits for switching mode power supply circuits.
2. Prior Art
Many devices are known in the art that provide start-up circuits for power supplies. However, the problem with those circuits is that they fail to protect the power supply circuitry from aberrations in the current being converted. Start-up circuits for switching mode power supplies are problematic because they must provide initial power to generate a modulated pulse to control a transistor power switch.
Power supplies take a current source and produce another current source of a different character. A common type of power supply is that which takes as input a high voltage and line frequency alternating current (common commercial and residential voltage of 110 V and 60 Hz of AC). The end result is a low voltage direct current (such as 12 V DC). At some stage of the conversion, the incoming current must be modulated to smooth the incoming current cycles to produce a substantially constant output current.
Switching mode power supplies use converters in conjunction with an oscillating means to regulate the current cycles. Switching mode power supplies are designed to convert input line frequency voltage in DC voltage, inverting that input to a high frequency AC voltage by a transistor power circuit. The transistor power circuit feeds the high frequency voltage through an output transformer, which then rectifies and filters it to produce the required DC voltage. Regulation of the output is accomplished by controlling the pulse width of the high frequency voltage wave. In order to run a switching mode power supply, a pulse generator must be used to generate a pulse width, and where the power supply is off-line, the pulse generator has no initial power to operate an oscillating means. Pulse generators provide periodic signals of varying widths to control the amount of current that is allowed to pass through the primary side winding(s) of the power supply's transformer(s). The width of the on and off signals are dynamically determined by the pulse generator according to the output of the power supply. Several prior art circuits provide start-up circuitry for initial generation of pulses but all fail to protect the entire power supply as well as the start-up circuitry from over stressed voltage conditions.
During the initial start-up, it is possible that power transformers and their transistor power switches will become stressed. Lack of initial duty cycle control creates that possibility when the transistor power switch operates with a duty cycle higher than 50% in the initial cycles. In forward or fly-back topologies, operating with a duty cycle of more than 50% causes the probability of power transformer saturation and power switch burn-out.
There are standard types of power conversion techniques known in the art. One is step-up conversion, which is a boost type conversion also known as a fly-back converter. Another is a step-down conversion, also known as a forward conversion. In the fly-back converter, energy is inductively stored during the transistor power-on periods, then, during the fly-back or transistor power-off periods, the inductor passes the stored energy to the load. In the forward converter, during the transistor power-on period, energy is stored in the power inductor and passes to the load. During the transistor power-off period, energy stored in the inductor continues to provide current flows to the load. All types of topologies generally require some sort of start-up circuitry to initiate the modulated pulse widths to control the transistor switch.
A start-up circuit is described in U.S. Pat. No. 4,063,307. This circuit is limited in application with fly-back converters. Also, this device is complicated, expensive, and unreliable.
Another start-up circuit is described in U.S. Pat. No. 4,246,634. One major problem with this circuit is that the pulse generator has no voltage insulation, with the pulse generation positioned on the primary side and exposed to potentially destructive high voltage surges. Second, the pulse generator monitors the primary side (input) voltage and not the secondary side (output) voltage.
Still another start-up circuit for a switching mode power supply is described in U.S. Pat. No. 4,695,936. In this device the pulse generator is located on the secondary side. The start-up circuit is located on the primary side, and it activates the switching transistor to start the power supply. The first embodiment described is a start-up circuit that continues to oscillate after the pulse generator commences to regulate. This arrangement is undesirable because the pulse generator has imperfect control over the oscillation means. Also, if the combination of pulse generator pulses and start-up circuit pulses create a duty cycle of over 50% then the transistor will begin to break down. The alternate embodiment described contains a circuit for disengaging the starting circuit, but the disengaging circuitry must be perfectly timed to work, and in practical terms, is too unreliable for use.
What is needed is a start-up circuit for a switching mode power supply that initially fully controls the start of modulated width pulses, then provides for a series of width modulated pulses to control the start of the power supply. Another object of the present invention is to provide a start-up circuit for a pulse generator which will automatically disengage after normal operation has been initiated.